1. Field of the Invention
This invention relates to fibers useful for reinforcing matrix materials, and more particularly to fibers having mechanically-created “bone shaped” morphologies characterized by weak surface interface bonding (allowing debonding & sliding) and locking end nodules which provide for load transfer along the length of the fiber, useful for enhanced strength, toughness, and overall performance in matrix materials such as asphalt, rubber, plastic, and particularly in such matrix materials as ready-mix concrete, shotcrete, and other hydratable cementitious compositions; and methods of forming such fibers.
2. Background of Prior Art
The fibers of the present invention are suitable for reinforcing a number of matrix materials, including asphalt, composites, plastics, etc. One of their primary intended uses is for reinforcing hydrated cementitious compositions such as ready-mix concrete, precast concrete, masonry concrete, shotcrete, bituminous concrete, gypsum and Portland cement-based composites, and other hydrated cementitious compositions. A major purpose of the fibers of the present invention is reinforcing concrete and concrete structures.
Concrete is typically a brittle material. When subjected to stresses exceeding its maximum tensile strength, cracks form and propagate in the concrete, eventually causing structural failure. The ability of a concrete structure to resist crack formation and propagation can be understood with reference to two properties: “strength” and “toughness.”
Concrete “strength” relates to the initial ability of a cement or concrete structure to resist crack formation. Concrete strength is proportional to the maximum load that can be sustained without cracking. It is a measurement of the minimum load required to initiate cracking. “Toughness” relates to the ability of the concrete to resist propagation (widening) of a crack once the crack has been initiated. Toughness is proportional to the force required to propagate or widen a crack after it is initiated. Toughness is determined by concurrently measuring: a) the load required to deflect a fiber-containing concrete once a crack has formed, and b) the amount of deflection.
Fibers designed to boost absolute toughness are not new to the composites industry. They belong to a class of fibers known as ‘macro’ fibers, based on their relatively large size (typically greater than 25 mm in length). Most such fibers depend on adhesion along the full length of the fiber. A smaller sub-group of fibers, including the class into which the current invention falls, depend on formations at the end of the fibers that mechanically lock into the matrix, preventing fiber pull-out, and thereby increasing overall toughness. Such fibers are generally known as “bone shaped fibers,” or BSF's. Los Alamos National Laboratory published literature on ‘bone shaped fiber’ morphology that inspired the inventor to develop the disclosed invention. Los Alamos National Laboratory abandoned fiber research due to loss of patentability and insufficient time to suggest production devices and processes. see Zhu-Byerlein [1] generally.
U.S. Pat. No. 3,953,953 (of Marsden) discloses a BSF-like fiber, describing fibers having “J”-shaped ends for resisting pull-out from concrete. One of the problems with stiff fibers having such physical deformities such as hooks or ripples is their propensity to form fiber to fiber entanglements, which poses problems in achieving uniform dispersion in a concrete mix, as well as problems with workability (slump). Other morphologies using crimped ends or ends with wave-like formations have similar problems.
The present invention is a BSF with a unique morphology comprised of a flattened rod (generally less than 50 mm long) terminated at each end by a prolate nodule. The flattened ‘nodule to nodule’ section of the rod is smooth and designed to be maximally free of adhesion to the host matrix. This allows for maximal crack bridging and load transfer over the entire length of the fiber. The overall shape of the prolate nodule resists mechanical fiber to fiber entanglement, resulting in it being more easily dispersed throughout a cementous hydrated concrete mix.
A uniformly flattened fiber (not a BSF) was disclosed by Rieder, et al., W. R. Grace U.S. Pat. No. 6,773,646. The fiber described in that invention was not a BSF, but was flattened over it's entire length, and micro-mechanically deformed for improved contact and adhesion with the matrix material. Although such deformation may improve pull-out resistance, the increased adhesion along the entire surface of the fiber results in a lack of load transfer over the entire length of the fiber, and fiber breakage at the crack interface, which in turn results less effective in crack bridging, load transfer, and overall toughness. Many other competitors add various wrinkles, abrasions, and other continuous modifications evenly distributed along the length of their fiber to encourage anchoring along the full length of he fibers to adhere to the matrix material, locking into the surface of the fiber. Fibers that depend on uniform ‘full length of fiber’ adhesion offer good initial crack mitigation up to certain levels of applied stress that is concentrated most often in a small zone along the fiber in the vicinity of the crack, but the subsequent rupture of the fiber will abruptly lower the toughness contribution that can be achieved by adhesion only methods. SI Concrete Systems (Synthetic Industries, Inc. USA), U.S. Pat. No. 5,981,630 (Banthia, et. al), acquired a crimped (rippled) sinusoidal patterned fiber that produces a controlled but somewhat less localized stress concentration at crack intersections than do the Reider (U.S. Pat. No. 6,773,646) or other straight fibers when mitigating cracking in a host matrix. The SI sinusoidal fiber is also continuously modulated along its full length. This fiber depends on localized stress concentration at points along the fiber length. Furthermore, the ‘wavy pattern’ leads to poor slump (workability), at high volume fractions, and ruptures versus pullout.
SI also obtained a ‘paddle fiber’, via Novocon, Inc., that is formed via a roll-forming flattened ends onto steel wire fibers. The paddle fiber is not a “bone shaped fiber” as described by Zhu-Byerlein, although designed to anchor at its ends in a manner similar to a BSF fiber. Many other fibers fall into the ‘paddle fiber’ anchor genera such as the Baeckert Dramix™ bent wire fiber, and the Royal Environmental. Inc. (Stacy, Minn., USA) bent wire fibers. The ‘paddle fiber’ patent expired recently thus the fiber and production methods are in the public domain. Bent wires encourage high stress concentrations at the bends thus encourage directional cracking, and the fibers sever upon post cracking induced failure in the matrix rather than pulling out with great difficulty while further not inducing energy uptake via the required uniform micro cracking.
In view of the disadvantages of the prior art as discussed above, the present inventors believe that a novel fiber for reinforcing matrix materials, and in particular hydratable cementitious materials such as concrete and shotcrete, are needed. Also needed are novel methods for making such fibers and for modifying such matrix materials.
The present invention improves the pull-out resistance of fibers from concrete while avoiding the kinds of mechanical or physical fiber attributes that might otherwise impede the ability of the fiber to be introduced into, and uniformly dispersed within, the concrete mix. The inventor applied artificial intelligence tools in determining the maximally effective geometry for the nodule-terminated, flat centered fibers described here. This geometry optimizes dispersability in the wet matrix material while maximizing the mechanical anchoring of the end nodule once the matrix has solidified.
One of the other impediments to the commercial production of viable BSF's is the inability to rapidly and reliably form the fibers while preserving the structural integrity and properties of the precursor material. The roller-former-cutter machine, which is the basis of the method of forming the fibers described herein, and is an integral component of my invention, solves this problem. The roll-forming machine receives as feedstock a plastic monofilament or metal wire precursor. The roll-former, in one embodiment, produces the fiber morphology described in the invention, i.e., flat in the middle and terminated at each end by a nodule. The nodules are formed by troughs in the surface of the two opposing rollers. The troughs run parallel to the rotational axis of the rollers. The nodules so formed retain for the most part the whole diameter or thickness of the precursor filament or wire, while the nodule-to-nodule connecting length is roll formed flat. The flattened aspect of the middle portion of the fiber also improves flexural properties, which in turn reduces fiber rupture failure when mitigating a crack which forms at an acute angle to fiber. Also embodied in the roll-former is a fiber cut-off component, consisting of a blade residing in the center of one of the nodule-forming troughs, and a corresponding anvil that resides in corresponding trough of the opposing roller. The invention enables roll-forming at high speed and low cost. No such roll-former exists that applies troughs machined in cylinders and fiber cut-off, concurrently. The combined roller-former and blade & anvil assembly roll against the precursor monofilaments (or other precursors) pinching them flat in the middle, forming nodules at the ends, and cutting to the fibers to length, leaving a nodule terminated fiber as a result.
3. Objects and Advantages
There are several objects and advantages of the disclosed invention. The described fiber morphology provides a multi-fold increase in toughness over competing fibers. Compared to other macrofibers whose toughness-increasing properties depend on surface area adhesion, the inventor's BSF's are designed to ‘slip’ away from the matrix along the length of their shank, thereby distributing force along their entire length, rather than only in the vicinity of crack formation. Considering two fibers with equal tensile strength, the fiber that distributes force along its entire length will be able to absorb much greater force than one that has the same force applied to the area near a crack. The result is increased post crack toughness of BSF's in general, and the disclosed fiber in particular.
Compared to BSF's in prior art, the inventor's fiber has several advantages. Fibers with hook-shaped ends, “J”-shaped ends, ripples, waves, twists, or other such deformities tend to tangle with one another, leading to problems in distribution throughout the matrix, along with problems in workability. The inventor's fiber achieves the mechanical anchoring necessary to anchor to the matrix, while not being shaped in such a matter as to cause them to ball up or tangle with one another.
The disclosed invention is a true bone-shaped-fiber, the nodules of which have a smaller cross sectional area than their flattened shanks. This provides a distinct advantage over paddle-shaped fibers, whose ends have a smaller cross sectional area than their shanks. While paddle shaped fibers may function when made of steele, when made of polyolefin materials the paddle shaped ends do not provide the necessary mechanical anchoring to maximize post crack toughness. The ability to utilize polyolefins and other hydrophobic precursor materials is a key to achieving even distribution throughout a cementitious mix. In addition, polyolefins and other such plastic fiber precursors have the required ability to de-bond and ‘slide’ from the matrix along their shanks, allowing for maximal crack bridging and load transfer over the entire length of the fiber, which in turn leads to the increased toughness of the fiber-filled matrix.
The manufacturing method utilizing the roller-former-cutters disclosed by the inventor are a novel and unique invention that make high speed, high volume formation of bone shaped fibers possible. Other methods disclosed in prior art are either impractical for high volume fiber production, do not form bone-shaped-fibers, and/or are not practical for use with polyolefin or other plastic monofilament precursor materials.
In view of the disadvantages of the prior art as discussed above, the present inventors believe that a new fiber for reinforcing matrix materials (particularly hydrated cements such as concrete and shotcrete) is needed, along with a method for manufacturing such fibers. The invention presents a simple, practical, method of reliably forming functional bone shaped reinforcement members from monofilament or wire precursor materials.